# std::signal

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Defined in header `<csignal>` |  |
/* signal-handler */* signal( int sig, /* signal-handler */* handler ); | (1) |
extern "C" using /* signal-handler */ = void(int); | (2) | (exposition only*)

Changes handling of the signal sig. Depending on handler, the signal can be ignored, set to default, or handled by a user-defined function.

When signal handler is set to a function and a signal occurs, it is implementation defined whether std::signal(sig, SIG_DFL) will be executed immediately before the start of signal handler. Also, the implementation can prevent some implementation-defined set of signals from occurring while the signal handler runs.

For some of the signals, the implementation may call std::signal(sig, SIG_IGN) at the startup of the program. For the rest, the implementation must call std::signal(sig, SIG_DFL).

(Note: POSIX introduced `sigaction` to standardize these implementation-defined behaviors)

### Parameters

sig | - | the signal to set the signal handler to. It can be an implementation-defined value or one of the following values:
SIGABRTSIGFPESIGILLSIGINTSIGSEGVSIGTERM | defines signal types (macro constant)
handler | - | the signal handler. This must be one of the following: SIG_DFL macro. The signal handler is set to default signal handler. SIG_IGN macro. The signal is ignored. A pointer to a function. The signature of the function must be equivalent to the following:
extern "C" void fun(int sig); |  |

### Return value

Previous signal handler on success or SIG_ERR on failure (setting a signal handler can be disabled on some implementations).

### Signal handler

The following limitations are imposed on the user-defined function that is installed as a signal handler.

If the signal handler is called NOT as a result of std::abort or std::raise (asynchronous signal), the behavior is undefined if the signal handler calls any function within the standard library, except std::abort std::_Exit std::quick_exit `std::signal` with the first argument being the number of the signal currently handled (async handler can re-register itself, but not other signals). the signal handler refers to any object with static storage duration that is not std::atomic or (since C++11)volatile std::sig_atomic_t. | (until C++17)
A plain lock-free atomic operation is an invocation of a function f from `<atomic>` or `<stdatomic.h>`(since C++23), such that: f is the function std::atomic_is_lock_free, f is the member function `is_lock_free` (e.g. `std::atomic::is_lock_free()`), f is a non-static member function of std::atomic_flag, f is a non-member function, and the first parameter of f has type cv std::atomic_flag*, f is a non-static member function invoked on an object obj, such that obj.is_lock_free() yields true, or f is a non-member function, and for every pointer-to-atomic argument arg passed to f, std::atomic_is_lock_free(arg) yields true. The behavior is undefined if any signal handler performs any of the following: call to any library function, except for plain lock-free atomic operations and the following signal-safe functions (note, in particular, dynamic allocation is not signal-safe): `std::signal` with the first argument being the number of the signal currently handled (signal handler can re-register itself, but not other signals). member functions of std::numeric_limits std::_Exit std::abort std::quick_exit The member functions of std::initializer_list and the `std::initializer_list` overloads of std::begin and std::end std::forward, std::move, std::move_if_noexcept All functions from `<type_traits>` std::memcpy and std::memmove access to an object with thread storage duration a `dynamic_cast` expression a `throw` expression entry to a `try block` initialization of a static variable that performs dynamic non-local initialization (including delayed until first ODR-use) waits for completion of initialization of any variable with static storage duration due to another thread concurrently initializing it | (since C++17)

If the user defined function returns when handling SIGFPE, SIGILL, SIGSEGV or any other implementation-defined signal specifying a computational exception, the behavior is undefined.

If the signal handler is called as a result of std::abort or std::raise (synchronous signal), the behavior is undefined if the signal handler calls std::raise.

On entry to the signal handler, the state of the floating-point environment and the values of all objects is unspecified, except for objects of type volatile std::sig_atomic_t
objects of lock-free std::atomic types side effects made visible through std::atomic_signal_fence | (since C++11)
On return from a signal handler, the value of any object modified by the signal handler that is not volatile std::sig_atomic_t or lock-free std::atomic is indeterminate. | (until C++14)
A call to the function `signal()` synchronizes-with any resulting invocation of the signal handler. If a signal handler is executed as a result of a call to std::raise (synchronously), then the execution of the handler is sequenced-after the invocation of `std::raise` and sequenced-before the return from it and runs on the same thread as std::raise. Execution of the handlers for other signals is unsequenced with respect to the rest of the program and runs on an unspecified thread. Two accesses to the same object of type volatile std::sig_atomic_t do not result in a data race if both occur in the same thread, even if one or more occurs in a signal handler. For each signal handler invocation, evaluations performed by the thread invoking a signal handler can be divided into two groups A and B, such that no evaluations in B happen-before evaluations in A, and the evaluations of such volatile std::sig_atomic_t objects take values as though all evaluations in A happened-before the execution of the signal handler and the execution of the signal handler happened-before all evaluations in B. | (since C++14)

### Notes

POSIX requires that `signal` is thread-safe, and specifies a list of async-signal-safe library functions that may be called from any signal handler.

Signal handlers are expected to have C linkage and, in general, only use the features from the common subset of C and C++. However, common implementations allow a function with C++ linkage to be used as a signal handler.

### Example

Run this code

    #include <csignal>
    #include <iostream>

    namespace
    {
        volatile std::sig_atomic_t gSignalStatus;
    }

    void signal_handler(int signal)
    {
        gSignalStatus = signal;
    }

    int main()
    {
        // Install a signal handler
        std::signal(SIGINT, signal_handler);

        std::cout << "SignalValue: " << gSignalStatus << '\n';
        std::cout << "Sending signal: " << SIGINT << '\n';
        std::raise(SIGINT);
        std::cout << "SignalValue: " << gSignalStatus << '\n';
    }

Possible output:

    SignalValue: 0
    Sending signal: 2
    SignalValue: 2

### References

- C++23 standard (ISO/IEC 14882:2024):

  - 17.13.5 Signal handlers [support.signal]

- C++20 standard (ISO/IEC 14882:2020):

  - 17.13.5 Signal handlers [support.signal]

- C++17 standard (ISO/IEC 14882:2017):

  - 21.10.4 Signal handlers [support.signal]

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 3756 | C++17 | it was unclear whether std::atomic_flag is signal-safe | it is

### See also

raise | runs the signal handler for particular signal (function)
atomic_signal_fence (C++11) | fence between a thread and a signal handler executed in the same thread (function)
C documentation for signal

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